This invention relates to optical fibers and more particularly to the use of an optical fiber for the detection of sound waves in a fluid medium.
Heretofore sound waves have been detected by different types of electromechanical devices such as capacitance microphones, and piezoelectroic and magnetostrictive transducers. These yield an electrical output signal when exposed to incident sound waves. Such devices provide a means for directly obtaining an electrical signal that varies in time in the same way that the pressure varies in the sound field. Some of the disadvantages of such systems are: they are bulky, expensive and require impedance matching between the sound detector and transmission and signal-processing system.
Optical waveguides and optical fibers have been used for conducting optical radiation from one point to another. Uses of such systems have been contemplated in the communication field for communicating optical signals and messages from one place to another. Other systems made use of stress applied to an optical fiber to modulate the optical radiation as it is transmitted through the fiber (for instance, see U.S. Pat. No. 4,068,191).
An article, "Optical Hydrophones for Sonar", by J. A. Bucaro, IEEE Publication 78 CH 1354-4AES, pp 298-302, September 1978, describes a fiber-optic hydrophone which uses a reference beam fiber and a sensing-beam fiber. The two beams are directed through the fibers, recombined, and allowed to interfere on the surface of a detector to detect acoustical energy incident on the sensing optic fiber. This subject matter has been set forth in Patent Application, Ser. No. 920,091, filed June 28, 1978, now U.S. Pat. No. 4,162,397.
Another system which makes use of an optical fiber coil through which optical radiation is transmitted is disclosed in U.S. Pat. No. 2,972,051. This patent is for the detection of ionizing radiation which permanently changes the optical index of the optical fiber. Further use of such a system has been set forth by George H. Segal et al. in an article "New Real Time dosimeters Use Fiber Optics" in Naval Research Laboratory Research and Development Highlights, Vol. 1, No. 1, pp. 7 and 8, December 1974.